1. Field of the Invention
The present invention relates generally to capacitive sensor arrays, and more particularly to a phase discriminating capacitive sensor array system having a capacitive sensor array producing sensor signals which are phase and amplitude controlled based on a frequency reference provided by a single frequency stabilized oscillator.
2. Description of the Related Art
Capacitive sensor arrays are used in industry to sense the proximity of an object. Conventionally, capacitive sensor arrays have been provided with a circuit having a number of free running oscillators, each corresponding to one of a number of sensor elements in the array provided to sense the object. When one of the sensors detected the object (was "pulled") the corresponding oscillator experienced a change in frequency. The conventional circuit recognized the object based on a change in capacitance caused by the change in the frequency of the free running oscillator. An example of such a conventional capacitive sensor array system is fully described in An In-situ Evaluation of a Caoacitive Sensor Based Safety System for Automotive Manufacturing Environment, by Don L. Millard, published in Robots 13, Conference Proceedings, SME/RI, May 7-11, 1989.
In the conventional capacitive sensor array system described above, however, because each of the free running oscillators provided for each sensor element is susceptible to being "pulled" as described above, the sensors have had to be placed far enough apart to ensure that the free running oscillators in each senor did not interact, couple and lock on to each other (cross talk). As a result, the range and sensitivity of the conventional capacitive sensor array system has been limited by a significant degree. Also, pixelized imaging has not been possible due to the lack of proximal collocation of the sensor elements.
Another disadvantage of the conventional capacitive sensor array system has been that the multiple free running oscillators have required that relatively wide band widths be provided for the sensor electronics input bandwidths. These wide bandwidths have resulted in increased noise and a decreased signal-to-noise ratio. As a result of this disadvantage as well, the range and sensitivity of the conventional capacitive sensor array system have been limited by a significant degree.
Still another disadvantage of the conventional capacitive sensor array system has been that frequency drift could not be prevented. Since the conventional capacitive sensor array system depended on "pulling" the oscillator, frequency stabilized oscillators, such as common crystal controlled oscillators, could not be provided. As a result, frequency drift has posed a major problem in the past, particularly due to increases in heat and temperature.
An additional disadvantage potentially present in a conventional capacitive sensor array system is that "capaciflector" sensors could not be easily employed. In a capaciflector sensor system, individual shields are employed for each sensor element. As a result, a capaciflector system using multiple free running oscillators must have sufficient spacing to prevent cross talk with the shields.